Field of the Invention
The invention relates to an electronic device having a semiconductor chip on a semiconductor chip connection plate, and to a leadframe having the semiconductor chip connection plate. The invention also relates to corresponding production methods.
If bond connections to a ground potential are additionally required by the semiconductor chip, it is possible to lead corresponding bonding wires from contact areas on the semiconductor chip, which are to be grounded, to the semiconductor chip connection plate. To that end, the semiconductor chip connection plate has a larger top side than the base area of the semiconductor chip. However, it is found that the reject rate is increased on statistical average by virtue of the fault rate in the case of devices with drawn-down additional ground lines to the semiconductor chip connection plate.
It is accordingly an object of the invention to investigate the cause of this increased reject rate and to avoid it to the greatest possible extent.
With the foregoing and other objects in view there is provided, in accordance with the invention, an electronic device having a semiconductor chip, a plastic housing and a leadframe for assembling the semiconductor chip. The semiconductor chip is mounted on a semiconductor chip connection plate of a flat conductor frame of the leadframe. The area of the semiconductor chip connection plate of the flat conductor frame is greater than the base area of the semiconductor chip. However, this larger semiconductor chip connection plate is structured in such a way that it has elongate openings along the edge sides of the semiconductor chip, so that the flat conductor frame has a ring that is defined by the elongate openings and that surrounds the semiconductor chip.
This electronic device has the advantage that the reject rate after the press molding of the plastic housing is lower than in previous constructions with a non-structured semiconductor chip connection plate having a larger top side than the base area of the semiconductor chip fitted thereon. Since the elongate openings surround the semiconductor chip, the bonded ground lines are arranged on the annular structure of the formed semiconductor chip connection plate. By virtue of the elongate openings between the ring structure and the semiconductor chip, the ring is mechanically decoupled from the semiconductor chip. Bond connections on the ring can shift with the ring and the pressed-on molding compound, with the result that the shear stress on the bond connection by the plastic molding compound during the press molding operation is reduced, and interruption or delamination of bond connections on the structured semiconductor chip connection plate is advantageously avoided in the region of the ring.
After the press molding operation, during the cooling of the plastic composition of the housing, the ring with the bond connections can follow the shrinking process of the plastic, with the result that the shear loads between the ring and the bond connection remain minimized in this phase as well. Without the invention""s openings all around the semiconductor chip, the shear loads between the bond connections and the semiconductor chip connection plate are in some instances so high that the bond connections tear and malfunctions of the electronic device thus occur. This problem is solved by the invention""s structuring of the semiconductor chip connection plate.
In order to ensure a ground line between the semiconductor chip connection plate and the surrounding ringxe2x80x94in one embodiment of the inventionxe2x80x94at least one electrically conductive bridge is provided between the ring of the semiconductor chip connection plate and the region of the semiconductor chip connection plate on which the semiconductor chip is arranged. The bond connections are arranged spaced apart from the mechanical bridge on regions of the ring that are mechanically decoupled from the region that carries the semiconductor chip. Hereinafter the region of the semiconductor chip connection plate on which the semiconductor chip is fitted is referred to as the island region or the island while the region of the semiconductor chip connection plate that is of annular design is defined as ring.
By virtue of the mechanical decoupling of the ring, which has bond connections to the ground connection, from the island region with the semiconductor chip, this ring region can be shifted and moved more freely in the molding compound. Relative movements between the bond connection and the ring are minimized so that, in the event of thermal cycling, delamination of the bond connection on the ring is avoided, and consequently, there is a reduction in the rejects in particular during the soldering test and after thermal cycling for the device. Although hitherto delamination of the chip on the island has necessarily led to delamination of the bond connections on the semiconductor chip connection plate, this inevitable coupling is overcome by the invention""s structuring of the semiconductor chip connection plate. In other words, even in the event of delamination of the chip in the island region, the ground connection is completely preserved via the bond connections and the ring, so that, after thermal cycling, the soldering and moisture test yields a significantly reduced reject rate.
In a further embodiment of the invention, the ring has a smaller thickness than the thickness of the island. Reducing the thickness of the ring reduces the stiffness thereof, and this minimizes the stresses, in particular shear stresses on account of relative movements between the island region of the molding compound and the ring with regard to the bond connection on the ring, and thus with regard to the ground line. Furthermore, on account of the smaller thickness of the ring, the ring with its bond connections can be completely embedded in the plastic compound. As a result, the region of the bond connections on the ring is better protected against mechanical stress including those from the outside, e.g. during the bending of the leadframe, during the cleaning of molding compound from the rear side or through stresses on account of the electronic device being pressed into a measurement receptacle, so that the reject rate which can occur during further processing likewise remains minimized.
A further embodiment of the invention provides for the semiconductor chip connection plate to be electrically connected to the ring and to be put at ground potential. To that end, provision is made of the mechanical bridge that connects the ring to the island.
A further embodiment of the invention provides for the semiconductor chip to have contact areas on its active top side. These contact areas are electrically connected to the ring via bond connections and are put at ground potential via the ring. Whereas on the island itself, the semiconductor chip with its passive rear side can be electrically connected to the island, and thus to ground potential directly by using a conductive adhesive or a soft solder, specific semiconductor structures on the active top side of the semiconductor chip cannot be connected directly to a ground potential. For these regions, corresponding contact areas are connected to the ring via bond connections, while the ring itself is in turn electrically grounded to the island using at least one electrically conductive mechanical bridge.
In one embodiment of the invention, therefore, it is provided that the ring is electrically connected to the semiconductor chip connection plate by at least one mechanical bridge located in the corner regions of the semiconductor chip connection plate. The bond connections are arranged in central regions of the elongate openings of the semiconductor chip connection plate. Providing the bond connections in the central regions of the elongate openings and fitting bridges only in the corner regions of the ring ensures mechanical decoupling of the central region of the ring, so that the bond connections in this region can be shifted in a relatively stress-free manner in the plastic housing.
A further embodiment of the invention provides for the ring to be electrically connected to the semiconductor chip connection plate by at least one mechanical bridge in its central regions. In this case, the bond connections are arranged in the corner regions of the semiconductor chip. This structuring of the semiconductor chip connection plate and the arrangement of the bond connections also means that the bond connections are fitted remote from the mechanical bridges. In this manner, the mechanical decoupling of the ring region in which the bond connections are situated reduces the shear stresses on the bond connections during thermal cycling and when plastic compound is pressed into the housing mold.
In a further embodiment of the invention, the flat conductor frame has flat conductor ends that are connected to contact areas on the semiconductor chip via bond connections. This second electrical path does not connect ground potential to the contact areas on the active top side of the semiconductor chip, but rather connects signal and supply lines to the semiconductor circuit on the active top side of the semiconductor chip. This electrical connection path is provided completely separately from the ground connection path using the ring. In combination with the ground connection via the ring, the flat conductor ends yield a complete supply for the integrated circuits on the active top side of the semiconductor chip and a complete access to the integrated circuits on the active top side of the semiconductor chip.
A further embodiment of the invention provides for the ring and the semiconductor chip connection plate to be offset in their height level by the thickness of the semiconductor chip relative to flat conductor ends. In this case, the semiconductor chip connection plate with the ring is arranged more deeply and the active top side of the semiconductor chip lies with its contact areas at the same height level as the top sides of the flat conductor ends. This embodiment of the invention also requires the bond connections to be drawn down from the top side of the semiconductor chip to the ring with the ground connection so that the bonding wires have to be fitted at different height levels. Nevertheless for the majority of the bonding wires that are to be connected to the flat conductor ends, an identical height level is produced, thus facilitating the production of the bond connections in this embodiment of the invention.
In a further embodiment of the invention, a difference between the thickness of the island and the thickness of the ring is realized by an etching-thin process. To that end, the regions of the leadframe that are not going to be etched thin can be covered, that is to say, in the island regions and the corresponding regions of the flat conductors, while the region of the ring and also the region of the bridges and the region of the flat conductor ends remain free and are exposed to an etching solution. This selective etching can actually be performed before the structuring of the flat conductor frame or of a leadframe strip. In a further step, the openings all around the region of the island and the openings between the flat conductors can be realized either by further selective etching or by stamping.
The etched-thin ring has the advantage that it can be completely embedded in a plastic compound in a preferred embodiment of the invention. To that end, the etching-thin process is carried out from the underside of the leadframe, and then during the introduction of the plastic compound, this etched-free region below the ring is then filled with plastic compound. In this case, the plastic compound can penetrate via the elongate openings to the underside of the ring and thus encloses the ring in plastic compound on all sides.
A further embodiment of the invention provides for the semiconductor chip and the semiconductor chip connection plate, with their side edges, to be surrounded on one side by a plastic molding compound. In this embodiment of the invention, the plastic compound is essentially applied from the top side of the leadframe strip, while the underside of the leadframe strip is provided with a supporting plate, so that no plastic is deposited on the underside of the island and the undersides of the flat conductors. Should plastic compound nevertheless penetrate to the underside of the island, then this portion of the plastic housing must be removed by complicated method steps in order to uncover the underside of the island and enable a ground connection. This ground connection can be effected by placing a conductive adhesive, for example, onto a metal area of a printed circuit board, or by soldering onto a metal area of a printed circuit board.
With the foregoing and other objects in view there is also provided, in accordance with the invention, a leadframe having a flat conductor frame. Such a leadframe is realized on a leadframe strip that receives both the individual semiconductor chips and the various bond connections. The leadframe has a semiconductor chip connection plate in its center. This semiconductor chip connection plate is structured, and has for its part, a ring connected to the semiconductor chip connection plate via at least one mechanical bridge. In this case, the semiconductor chip connection plate with its ring is provided for a ground connection. From the ring, it is possible to produce bond connections to the active top side of the semiconductor chip, while the passive rear side of the semiconductor chip lies on the top side of the semiconductor chip connection plate.
Such a leadframe has the advantage that, on the ring, as many bond connections as are desired can be accommodated as ground lines for the integrated circuits on the active top side of the semiconductor chip. By virtue of the mechanical decoupling of the ring from the semiconductor chip, the bond connections with the plastic molding compound can be shifted together with the ring, without shear stresses resulting in tearing between the bond connection and the ground connection.
In a further embodiment of the invention, in order to magnify the mechanical decoupling and to increase the elasticity of the ring, the leadframe has a ring with a smaller thickness than the thickness of the semiconductor chip connection plate in the region of the semiconductor chip. To that end, the semiconductor chip connection plate can be etched thin in the region of the ring. Both the semiconductor chip connection plate and the ring are electrically connected and grounded, with the result that the entire passive rear side of the semiconductor chip can be connected to ground. In addition, on the active top side of the semiconductor chip, selected regions can be connected to the ring of the leadframe via bond connections.
In one embodiment of the leadframe, the ring is electrically connected to the semiconductor chip connection plate by a mechanical bridge in the corner regions of the semiconductor chip connection plate. This embodiment has the advantage that the inherently stiff corner regions of the ring are now additionally utilized in order to fit the stiffening bridges there, so that the central region of the ring of this leadframe remains reserved for possible bond connections, which are then mechanically decoupled from the semiconductor chip.
In another embodiment of the invention, at least one mechanical bridge is arranged in a central region, so that the entire remaining region of the ring is available for bond connections. While the ring can be used to provide additional ground lines to the active top side of the semiconductor chip, signal access and supply lines are made available via the flat conductor ends of the flat conductor frame.
In a further embodiment of the invention, the flat conductor ends have a bondable layer for the possible bond connections. This bondable layer may be a silver layer or a gold layer or else a nickel layer or a layer made of alloys of these metals.
In a further embodiment of the invention, height or level of the ring and the remainder of the semiconductor chip connection plate are arranged offset relative to the height or level of the flat conductor ends of the flat conductor frame. By virtue of this offset arrangement, the semiconductor chip connection plate and the ring lies deeper than the top sides of the flat conductor ends. This facilitates the bonding connection of the flat conductor ends to the contact areas on the active top side of the semiconductor chip. The difficulties of the unequal level between top side of the semiconductor chip and the ground contact on the ring remain unchanged, however, when creating bond connections toward the ground potential.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for producing an electronic device having a semiconductor chip that is mounted on a semiconductor chip connection plate of a flat conductor frame of a leadframe. The flat conductor frame has a ring. The method includes the following steps:
providing a leadframe with a flat conductor frame in the center of which there is arranged a semiconductor chip connection plate whose top side area is greater than the base area of a semiconductor chip;
introducing elongate openings, which surround the base area of the semiconductor chip, into the semiconductor chip connection plate for the purpose of forming a ring;
applying a semiconductor chip to the semiconductor chip connection plate;
producing bond connections between the ring and the contact areas on the semiconductor chip;
producing bond connections as signal or supply lines between flat conductor ends and the contact areas on the active top side of the semiconductor chip; and
packaging the flat conductor frame with the semiconductor chip into a plastic molding compound of a plastic housing while embedding the bond connections.
This method provides an electronic device that enables an improved production rate of fault-free devices to be obtained, since tearing of the ground lines on the semiconductor chip connection plate is reduced. This is because the ground lines in the form of bond connections are now effected on the structured semiconductor chip connection plate in a ring region and not on the island region on which the semiconductor chip is arranged.
In an exemplary implementation of the method a further method step is provided, in which the semiconductor chip connection plate is selectively etched thin in the region of the ring that will be formed. This selective etching is performed before the elongate openings are introduced. This etching-thin process can also be extended to the flat conductor ends of the flat conductor frame, with the result that, at the same time, thin flat conductor ends are also produced whose thickness corresponds to the thickness of the etched-thin ring. This etching-thin of the ring imparts a higher compliance to the ring. The result is that the shifts produced by thermal cycling, and in particular, during the process for pressing in the plastic molding compound of the housing can be absorbed without the occurrence of shear stress on the bond connection of the ring with the ground connection.
In a further embodiment of the invention, the etched-thin ring is completely embedded in a plastic compound. This has the advantage that the bond connections arranged on the ring are completely protected against being influenced and damaged. With the foregoing and other objects in view there is provided, in accordance with the invention, a further method for producing a leadframe with a flat conductor frame having a center formed with a semiconductor chip connection plate with a ring. The method has the following method steps:
providing a leadframe with a flat conductor frame in the center of which there is arranged a semiconductor chip connection plate whose top side area is greater than the base area of a semiconductor chip; and
introducing elongate openings, which surround the base area of the semiconductor chip, into the semiconductor chip connection plate for the purpose of forming a ring while maintaining at least one mechanical bridge between the ring and the remaining semiconductor chip connection plate.
The production of such a lead frame has the advantage that the previously rigidly connected area providing two functions is now functionally and mechanically separated. First, a ground connection for the rear side of the semiconductor chip, and second the ground connection of the bond connections to the top side of the semiconductor chip are now functionally and mechanically separated. This produces an island for receiving the semiconductor chip and a ringxe2x80x94surrounding the island for receiving the bond connections through the structuring of the semiconductor chip connection plate. The requisite openings all around the island for the semiconductor chip can be introduced by using an etching operation. This etching operation can be effected wet-chemically or else by dry etching. A stamping possibility for structuring the semiconductor chip connection plate is also possible, but stamping yields smooth cutting areas, while in many applications it is advantageous to produce etched through openings since the etched profiles can be anchored better in the plastic compound of the housing.
In a further exemplary implementation of the method, the region of the flat conductor frame in which the ring will be formed is etched thin. This etching-thin of a region of the semiconductor chip connection plate can be effected before introducing the elongate openings. This has the advantage that the openings can be etched by merely penetrating through the residual thickness of the material of the semiconductor chip connection plate in order to form the ring. At the same time as the etching-thin of the region of the ring, a region of the flat conductor ends of the flat conductor frame can also be effected. As a result, the flat conductor ends also acquires a greater and higher elasticity, and consequently, their bond connections likewise achieve greater flexibility with respect to the loads due to the plastic molding compound or due to thermal cycling. Furthermore, it is possible to realize smaller distances between the flat conductor ends on account of the reduced thickness of the flat conductor ends.
To summarize, it can be stated that housings with an xe2x80x9cexposed die padxe2x80x9d or an externally accessible semiconductor chip connection plate very often have a delamination between the plastic molding and the region of the semiconductor chip connection plate which is not covered by the chip. In other words, the delamination essentially relates to the bond connections that are additionally fitted on the large-area semiconductor chip connection plate. The uncovered region of the semiconductor chip connection plate is only encapsulated on one side by the plastic molding compound, and thus produces a mechanical stress on the bond connections arranged in this region, since a poor interfacial adhesion of the plastic molding compound occurs in this uncovered region of the semiconductor chip connection plate. As a result of the delamination of the plastic molding compound from the top side of the semiconductor chip connection plate in the uncovered region, the previously produced bond connections are sheared off on account of the high shear loading. Down bonds (bonding wire connection between the chip and the plate) and/or ground bonds (bonding wire connections between the plate and external flat conductors) are to be fitted on this, usually delaminated, part of the semiconductor chip connection plate. The delamination in this region, in conjunction with the thermal stresses occurring during soldering and/or during thermal cycling, leads to relative movements between the plate and the molding compound in all three spatial directions, so that this leads partially or completely to the tearing away of the bond connections on this plate. Since this delamination, in the case of down or ground bonds (drawn-down bond connections or ground lines), is a decisive criterion for reaching a specific soldering level during the so-called MSL test (moisture level test), housings with an xe2x80x9cexposed die padxe2x80x9d and with semiconductor chips with ground bonds/down bonds enable only very poor soldering levels, particularly in the case of large housings, i.e. also large semiconductor chip connection plates.
A significant indication as to how it is possible to solve this problem of delamination, particularly in the MSL test and the associated high failure rate of the devices with a large semiconductor chip connection plate in relation to the base area of the semiconductor chip, is afforded by the bond connections with the flat conductor ends. By way of example, in a VQFN housing (very thin quadflat nonlead packages) the flat conductor connections likewise carry bond connections in the plastic molding compound, which are embedded completely independently of the flat conductor frame in the molding compound. These bond connections follow the movements of the molding compound during all thermal cycling processes and also during soldering as well as during the soldering and moisture test, without the occurrence of damage to the bond connections. It was during the observation of this phenomenon that the idea arose for solving the problem of the consequences of a delamination of the plastic compound on the uncovered semiconductor chip connection plate that must partly carry bond connections.
The invention""s mechanical decoupling of the region of the semiconductor chip connection plate with the down bonds/ground bonds situated thereon from the island with the semiconductor chip, means that the endangered region with the down bonds/ground bonds can move more freely and with less hindrance in the molding compound, without relative movements occurring between down bonds/ground bonds and the inventive ring.
Furthermore, the mechanical decoupling of the down bond/ground bond region from the island with the semiconductor chip means that a delamination of the island region with the chip cannot inevitably lead to a delamination of the region with the down bonds and ground bonds.
If delamination of the region with the down bonds/ground bonds nevertheless occurs, this region can adapt itself to the movements of the molding compound. Stresses due to relative movements are thus reduced in the bond connection, and the down bonds/ground bonds are not damaged. xe2x80x9cHalf etchingxe2x80x9d (etching thin) to half of the thickness of the semiconductor chip connection plate in the region with the down bonds/ground bonds reduces the stiffness of the ring, and minimizes the stresses and also the relative movements between this ring and the molding compound.
The xe2x80x9chalf etchingxe2x80x9d of the region with the down bonds/ground bonds results in a complete encapsulation of this region with the molding compound during the packaging to form the electronic device. As a result, this region is significantly better protected against mechanical stress from outside, as occurs e.g. through bending of the lead frames, through deflashing (elimination of excess plastic molding compound) or through stress occurring during the introduction into a measurement receptacle. This in turn leads to the minimization of the loading on the sensitive down bonds/ground bonds.
xe2x80x9cDouble-ring platingxe2x80x9d (double metal plating) of the xe2x80x9cinner leadsxe2x80x9d (of the flat conductors) and of the ring with the down bonds, and dispensing with silver-plating of the island with the semiconductor chip make it possible to increase the molding compound adhesion on the actual semiconductor chip island especially as the introduced elongate openings also now completely encapsulate the semiconductor chip island laterally with the plastic molding compound. In this case, the position of the down bonds/ground bonds in the ring region can be chosen as desired, as long as a sufficient distance is maintained from the mechanical bridges between the ring and the semiconductor chip island. This leads to an improvement of the housing during the MSL test.
The advantage of the invention essentially consists in the fact that this new housing construction eliminates the critical, reliability-reducing fault mechanisms of tearing the down and ground bonds after an MSL test or TC loading (temperature cycle loading) by using the inventive flat conductor frame construction. This elimination is effected by mechanically decoupling the region with the down bonds/ground bonds from the island with the semiconductor chip. Simultaneous stress reduction and a better mechanical anchoring due to the xe2x80x9chalf etchingxe2x80x9d is also achieved.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an electronic device having a semiconductor chip on a semiconductor chip connection plate and method for producing it, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.